issue #118: log exceptions for emulated commands, fix AttributeError in helpers.py

Turns out Ansible can't be trusted to actually check the result
dictionary everywhere it expects one, so put the real exception text
into -vvv output too.

ansible_mitogen/helpers.py

@@ -289,7 +289,7 @@ def set_file_mode(path, spec):
     """
     mode = os.stat(path).st_mode
 
-    if spec.is_digit():
+    if spec.isdigit():
         new_mode = int(spec, 8)
     else:
         new_mode = apply_mode_spec(spec, mode)

ansible_mitogen/mixins.py

@@ -28,6 +28,7 @@
 
 from __future__ import absolute_import
 import commands
+import logging
 import os
 import pwd
 import shutil
@@ -52,6 +53,9 @@ import ansible_mitogen.helpers
 from ansible.module_utils._text import to_text
 
 
+LOG = logging.getLogger(__name__)
+
+
 def get_command_module_name(module_name):
     """
     Given the name of an Ansible command module, return its canonical module
@@ -157,6 +161,7 @@ class ActionModuleMixin(ansible.plugins.action.ActionBase):
             if stdout:
                 dct['stdout'] = repr(rc)
         except mitogen.core.CallError:
+            LOG.exception('While emulating a shell command')
             dct['rc'] = 1
             dct['stderr'] = traceback.format_exc()